The present invention relates to a method for producing decorations which are essentially color-stable in a porcelain firing (glost firing), that is, decorations which endure the thermal stress of the porcelain firing without significant color changes after the application of the decoration onto unfired porcelain and the application of a glaze onto the decorated surface. The decorations contain inclusion pigments based on crystals enclosed in a transparent oxidic or silicate casing and which consists of colored heavy-metal sulfides, selenides or sulfoselenides, especially zirconium inclusion pigments containing cadmium sulfide or cadmium sulfoselenide. Further subject matter of the present invention relates to a pigment composition suitable for carrying out this method which contains at least one color-stabilizing auxiliary agent in addition to one of the cited inclusion pigments.
Various methods are known for the production of colored decorations on porcelain. Thus, glost-fired, glazed porcelain can be subsequently decorated using ceramic coloring bodies by applying a decorative layer by means of customary decorative methods and firing on the obtained decoration in a so-called decoration firing. Another method for producing decorations on porcelain involves porcelain which is pre-fired in a so-called biscuit firing at approximately 1000.degree. C., and is pre-strengthened or hardened in this manner, and which is converted after application of a colored decoration and a porcelain glaze during the subsequent actual porcelain firing at 1200.degree. to 1500.degree. C. (usually designated as a glost firing or final firing) into the actual glazed and decorated porcelain. Soft porcelains can be provided with a colored decoration more easily than hard porcelains since a greater number of ceramic coloring bodies endure the lower firing temperature (1200.degree. to approximately 1350.degree. C. possible in the case of soft porcelain) without decomposition or appreciable color change. Under the extreme temperature stress of porcelain firing for hard porcelain in particular (i.e., several hours in a range of approximately 1350.degree. C. to approximately 1500.degree. C.), most traditional coloring bodies, including inclusion pigments, are no longer color-stable or no longer sufficiently color stable. In such instances the decorative possibilities are limited.
Attempts have been made in recent porcelain finishing methods to produce a glazed porcelain provided with a colored decoration in such a manner that only a single firing procedure, a so-called single rapid firing, is necessary. The firing temperatures are usually around 1400.degree. C. As has already been explained above, the limited palette of available color-stable pigments is also noticeable here in a negative fashion. There is a report about the production of porcelain according to this new technique in "Keramische Zeitschrift" 43, No. 7 (1991), pp. 473-477. Suggestions about glazing errors occurring in this technique and possibilities of eliminating them (e.g., an oxidizing firing atmosphere at 1280.degree. C. before the melting fire) can be gathered from cfi/Ber. DKG 2/82, pp. 142-148.
Even the inclusion pigments based on crystals of colored heavy-metal sulfides, selenides or sulfoselenides enclosed in a transparent oxidic or silicate casing, which pigments are known to be especially temperature-stable and glaze-stable, prove to be insufficiently color-stable under the conditions of a porcelain firing at 1350.degree. to 1500.degree. C. For example, gray and/or brown discolorations appear in zirconium-cadmium sulfide yellow and zirconium-cadmium sulfoselenide red. It is theorized that under the conditions of a porcelain firing the chromophores cadmium sulfide and cadmium sulfoselenide partially exit from the casing surrounding them and separate colloidally in the glaze in this form or in combination with glaze components.
The present invention sought to solve the problem of developing a method for the production of decorations which are sufficiently color-stable in a porcelain firing (glost firing) without having to develop basically new pigments. There was particular interest in making available inclusion pigments with sulfidic and/or selenidic inclusions ranging in color from bright yellow to red, like those known from DE-OS 23 12 535 and commercially available, for the decoration of porcelain before the actual porcelain firing. Finally, a further problem sought to be solved concerns making available a pigment composition containing a color-stabilizing auxiliary agent in addition to the cited inclusion pigments so that the pigment composition can be readily used for the production of decorations which are color-stable in a porcelain firing.